1. Field of the Invention
The present invention relates to a method of producing a sliding contact which is formed of a resilient strip and a noble metal ball and slides on a sliding substrate.
2. Description of the Related Art
The sliding contact shown in FIG. 12 has been known as a sliding contact for encoders used for so-called mouses. This type of sliding contact is supported by the supporting body 2 and is formed of a plurality of resilient strips 3 and a noble metal strip 4 of wear resistance. The supporting body 2 moves on a sliding substrate, for example, resistor 1 (or, a pulse switch substrate) in the direction shown with the arrow A in FIG. 12. The resilient strips 3 each have the front end extending to the resistor 1. The noble metal strip 4 is connected to the front end of the resilient strip 3 and has its front end contacting to the resistor 1. The resilient strips 3 are arranged in parallel so as to be perpendicular to the traveling direction of the supporting body 2.
However, the conventional sliding contact has the following disadvantages:
1) The processing is complicated since a large number of noble metal strips 4 must be arranged in line and bent portions are needed to contact with the resistor 1. PA1 2) In order to obtain a sufficient contact strength between the noble metal strip 4 and the resilient strip 3, it is required to set large contacting area of the noble metal strip 4 and the length of the resilient strip 3 is relatively short. Therefore the pressure of the noble metal strip 4 against the resistor 1 is unstable because the degree of freedom of the resilient deformation of the resilient strip 4 is small, whereby the reliability as a contact is poor. PA1 3) The manufacturing cost is high because the noble metal strip 4 requires a relatively large area other than the contact point and is formed of a special noble metal material having wear resistance and elastic properties.
The sliding contacts shown in FIGS. 13 to 15 have been proposed in order to solve the above problems.
FIG. 13 is a side view showing a sliding contact according to the proposal. FIG. 14 is a side view showing the state of making the main portion of the sliding contact in FIG. 13. FIG. 15 is a front view showing the state of making the main portion of the sliding contact in FIG. 13. In FIG. 13, like numerals are given to those identical to elements shown in FIG. 12. That is, numeral 1 represents a resistor and 2 represents a supporting body.
The sliding contact, as shown in FIG. 13, is constituted of a resilient strip 5 having its front end supported by the supporting body 2 and welded to the resilient strip 5, and a a noble metal ball 6 of wear resistant property contacting to the resistor 1. The resilient strip 5 is formed of, for example, a resilient material such as german silver, phosphor-bronze, or the like. The noble metal ball 6 is formed of, for example, Pt-series noble metal material, or Pd-series noble metal material.
In the sliding contact, when the noble metal ball 6, as shown in FIG. 14, is welded to the resilient strip 5, is positioned by arranging in the recess portion 7a formed in the jig 7. The front end of the resilient strip 5 is placed on the noble metal ball 6. Then a so-called beam welding is performed by irradiating a YAG laser beam 8 onto the resilient strip 5. As a result, the resilient strip 5 is fused by the irradiation heat due to the laser beam 8 while the noble metal ball 6 is fused by the the heat conducted through the contact point P, whereby the resilient strip 5 is welded with the portion at the contact point P.
According to the above sliding contact, the resilient strip 5 is point-contacted with the noble metal ball 6 and there is an air gap between the resilient strip 5 and the noble metal ball 6 except for the contact point P. Therefore there has been a disadvantage in that if the irradiating position of the YAG laser beam 8 shifts somewhat from the top of the contact point P, the air gap 9 insulates the irradiation heat of the laser beam 8, thus causing insufficient fusion of the noble metal ball 6. As a result, the weld strength between the resilient strip 5 and the noble metal ball 6 is decreased largely.
FIG. 16 is a characteristic diagram showing the experimental correlation between laser beam irradiation position and weld strength obtained by the present inventor. In the Figure, the ordinate axis shows the weld strength between the resilient strip 5 and the noble metal ball 6 and the transverse axis shows irradiation positions of the YAG laser beam 8. In the irradiation position, the reference position (0) is one running through the contact point P and the center C of the noble metal ball 6. The positive shift (+) shows the irradiation position shifted in the elongate direction of the resilient strip 5 (to the right side in FIG. 13). The negative shift (-) shows the irradiation position shifted toward the front end of the resilient strip 5 (to the left side in FIG. 13).
In comparison with mean values shown with black dots in FIG. 16, for example, when YAG laser beam 8 is irradiated onto the top of the contact point P, or, at the reference position (0), the weld strength is 1858.25 gf. However if the irradiation position of the YAG laser beam 8 is shifted in the elongate direction or toward opposite side (-) of the resilient strip 5 by 0.05 mm, the weld strength decreases to 1676.5 gf. In the similar manner, if the YAG laser beam is shifted toward the front side (+) of the resilient strip 5 by 0.05 mm, the weld strength decreases to 1418.17 gf. Therefore, there has been a disadvantage in that when the irradiation position of the YAG laser beam 8 is shifted somewhat from the contact point P, the weld strength between the resilient strip 5 and the noble metal ball 6 reduces largely, as described above.
Furthermore, according to the above-mentioned sliding contact, when the YAG laser beam 8, as shown in FIG. 15, is irradiated onto the resilient strip 5, it may hit erroneously the jig 7 by deflecting out of the side end of the resilient strip 5, whereby the jig 7 may be partially damaged due to the irradiation heat of the YAG laser beam 8.